As it turns out, the OAuth entry URL I was using was missing a pair of square brackets.I was usinghttps://{shop}/admin/oauth/authorize?client_id={api_key}&scope={scopes}&redirect_uri={redirect_uri}&state={nonce}&grant_options=valueinstead ofhttps://{...

I don't know if this extra bit of context helps, but when this started happening I tried a few times in Incognito mode to avoid any sticky state. No effect unfortunately.

I have a test app and a dev store. In the beginning of the OAuth flow, when I try to redirect to the URL for "Showing the installation permissions prompt", I get immediately redirected to https://partners.shopify.com/. There's no intermediate route; ...